In initial studies, we found a new, distinctive, low-amplitude, high-frequency signal late in the QRS complex in 92% of patients with ventricular tachycardia (VT) after myocardial infarction (MI). The late QRS abnormality required computer signal averaging and a newly developed digital filter to detect. Only 7% of patients with benign rhythm disorders after MI had the abnormality. The purpose of the proposed research is to establish the incidence and significance of the late QRS abnormality among patients with and without ventricular arrhythmias. Three groups of patients will be studied: a) patients with VT or ventricular fibrillation (VF) who are undergoing electrophysiologic testing for diagnosis and therapy; b) patients with prior MI; and c) normals. The specific aims are to establish whether the QRS abnormality identifies patients with VT/VF from all patients after MI, the clinical and physiologic significance of the signal in patients with VT/VF, and whether antiarrhythmic therapy (medical and surgical) alters the late QRS abnormality. The incidence of the late QRS abnormality in patients with prior MI and the relation of it to rhythm status will be studied. In addition, we will investigate the incidence of the abnormality following acute MI and the relationship to mortality and sudden death over the next year. In brief, the proposed research is designed to evaluate whether a new noninvasive test, based on advanced signal processing, is a reliable marker for VT and VF in man.